Time Allowed: Three
Horus Maximum marks: 200
SECTION B
Candidates should attempt Questions 1 and 5 which are
compulsory and any THREE of the remaining questions selecting at least ONE
question from each Section.
If any data is considered insufficient, assume suitable
valur.
SECTION B
5.
Answer any four parts:
(a)
The evaporator and condenser temperatures in a
reverse Carnot refrigeration cycle of 1 TR capacity are 263 K and 313LK
respectively. The outlet of compression is saturated vapour and into to turbine
is saturated liquid. Find the mass flow rate, work done condenser heat
rejection and COP. Properties of refrigerant at saturation in SI units are as
follows:
|
t
(K)
|
hf
|
hg
|
S
f
|
Sg
|
|
263
|
154.056
|
1450.22
|
0.82965
|
5.755
|
|
313
|
390.587
|
1490.42
|
1.64377
|
5.1558
|
(b)
Air enters a cooling coil at 30˚C, 75 % relative
humidity. The apparatus dewpoint is 12˚ c and bypass factor is 0.15. Find the
temperature and humidity ratio at outlet of cooling coil. If mass flow rate of
air is 10 kg / s, find the condensate rate and cooling capacity of cooling
coil. The partial pressures of water vapour at 12˚ C and 30˚C are 1.4017 and
4.2431 kPa respectively. Atmospheric pressure is 101.325 kPa. Enthalpy of
condensate at 12˚C = 50.24 kJ / kg.
(c)
Explain the nomenclature Rabc for CFCs and
inorganic compounds. What is meant by ozone depletion? Name at least two
refrigerants that do not cause it.
(d)
State Buckingham’s 
theorem. Using Buckingham’s theorem obtain an expression for drag force on
a partially submerged body moving with a relative velocity V in fluid, the
other variables being linear dimension L height of surface roughness K, the
fluid density ƿ and gravitational accelerating.
theorem. Using Buckingham’s theorem obtain an expression for drag force on
a partially submerged body moving with a relative velocity V in fluid, the
other variables being linear dimension L height of surface roughness K, the
fluid density ƿ and gravitational accelerating.
(e)
Discuss the criteria for the selection of site
for steam and hydroelectric power plants.
6.
(a) A refrigeration system of 10 TR cooling
capacity has condenser and evaporator temperatures of 45˚C and – 20 ˚C
respectively. The vapour leaving the evaporator sub cools the liquid leaving
the condenser from 45 ˚C to 25˚C. Draw schematic diagram and T.S fiagram
considering isentropic compression, isobaric heat absorption and rejection.
Determine mass flow rate, compressor work. Condenser heat rejection and COP.
Use vapour specific heat at condenser pressure to find COP. USE vapour specific
heat at condenser pressure to fin adiabatic discharge temperature and enthalpy.
The properties in SI units at saturation are:
|
|
hf
|
hg
|
sg
|
C pg
|
Cpf
|
|
-20˚ C
|
17.8
|
178.7
|
0.7088
|
0.61
|
-
|
|
45˚C
|
79.7
|
204.9
|
0.6812
|
0.755
|
1.02
|
(b) Define thermodynamic wet bulb
temperature t* and show that humidity ratio may be expressed as
W = W* h* fg - 1.005 (t- t*) /
hg (t) – h*f
Where enthalpy of moist air is expressed as
h= 1.005 (t) + hg (t).
7.
(a)
Diabatic flow of dry air takes place through a frictionless constant
area duct. At some particular section of the duct, the Mach number is 4.0 while
stagnation temperature and static pressure are 280 K and 0.5 bar respectively.
Calculate the stagnation temperature. Static and stagnation pressures at a
section where the Mach numbers is 2.0 also find the amount of heat transfer
which causes this reduction Mach number. Take Cp= 1.005 kJ/ kg and γ = 1.4.
|
M
|
P
/ᵖP*
|
T/
T*
|
Tₒ/
Tₒ*
|
Pₒ
/ Pₒ*
|
|
2.0
|
0.364
|
0.529
|
0.793
|
1.503
|
|
4.0
|
0.1026
|
0.168
|
0.589
|
8.277
|
(b) Explain what you understand by specific speed of a turbo machine.
Give its importance.
Calculate the specific speeds of the following cases:
(i)
A 2500 kW gas turbine is running at a speed
of 18000 RPM. The entry and exit
conditions of the gas are T₁ = 1100 K₁ P₁ =60 bar, P₂ = 30 bar.
(ii)
A centrifugal compressor develops a pressure
ratio of 1.5 while running at 24000 RPM and discharging 2.0 kg/s of air, the
entry conditions are P₁ =1.0, T₁ = 290K.
1 or both cases take
γ + 1.4, R = 287 J / kg – k, Cᵨ =1.005 kJ / kg –k.
8.
(a) Explain clearly, “heat rate curve’ and “
Incremental rate curve’. Show that the incremental rate curve crosses the heat
rate curve at the lowest value of heat rate.
The incremental fuel costs for two
generating units A and B of a plant are given by
dF a / dPa
= 0.065 Pa + 25
dF b / dPb = 0.08 Pb + 20
where F is fuel cost in Re / hr and P is
power output in MW. Find –
(i)
The economic loading of the two units when the
total load supplied by the power plant is 160 MW.
(ii)
The loss in fuel cost/ hr if the load is shared
equally by the units.
(b) Discuss the importance of the terms, capacity factor and use factor
from the economic point of view of the power plant.
A power station is said to have a sue factor of 50% and a capacity factor
of 45%. How many hours the plant did not operate during the year?
